1. Field of the Invention
The plastic pipe coupling apparatus and method of the present invention relates to couplings for fastening plastic pipes together without necessity of forming coupling threads in the pipe or incurring the disadvantages attendent direct contact between metallic fittings and corrosive elements, such as soil into which a coupled pipe might be buried.
2. Description of the Prior Art
It is well known in the art that plastic pipe, such as polyethelene, has become a popular conduit for underground transmission of natural gas. As use of such polyethelene pipe has become more and more popular in industry, the necessity of perfecting a coupling method has become apparent. In response to that demand, numerous different coupling procedures have been proposed. Some coupling procedures contemplate the actual fusion of the plastic pipe to form a gas tight coupling. Other devices have been proposed which contemplate use of telescopically interfitting parts having dynamic seals, such as O-rings, interposed therebetween.
Representative of some of the efforts to provide a satisfactory plastic pipe coupling is the polyester fittings formed with a tubular body having coupling members bonded to the opposite ends thereof with one such coupling member being formed with interior seals which seal dynamically with a tube received telescopically within the tubular body. A device of this type is shown in U.S. Pat. No. 4,023,831. Devices of this type, however, suffer the shortcoming that the seals are exposed directly to the telescopical tube and to any foreign matter, dirt or debris which may be carried on such tubes. Moreover, the seals, after long years of use, tend to cold flow out of their retaining glands thus breaking the sealing engagement with the pipe.
A dynamic plastic pipe coupling is shown in U.S. Pat. No. 4,386,796 issued on the application of applicant Robert W. Lyall and Donald C. Hill. This coupling, while being satisfactory for underground repair work, does suffer the shortcoming that it incorporates dynamic seals, requires heat fusion and some technical competency to properly install the coupling.
It has been thought by those skilled in the art that an economical plastic fitting could not be formed by mere axial sliding of a completion sleeve onto a fitting to compress the wall of the plastic pipe into grooves formed in a plastic spigot. Metal to plastic connections have been proposed which include metal inserts for insertion into the end of a plastic pipe and over which a metal finishing sleeve might be drawn to compress the wall of the pipe radially inwardly into annular, flat bottom, grooves formed in the exterior wall of an insert. A device of this type is shown in U.S. Pat. No. 4,293,147. Such devices, while satisfactory for their intended purposes, suffer the shortcoming that they incorporate metal sleeves and flanges which, when the pipe is buried, create an electrolytic circuit which severely attacks the metal fitting and sleeve.
Other devices have been proposed which incorporate double ended spigots having a profile which is somewhat saw tooth shaped for insertion into a thermo-plastic pipe and over which a shrinkable sleeve may be received for shrinkage onto the pipe to compress it against the grooves formed in the spigot. A device of this type is shown in Australian Pat. No. 220,055. Devices of this type have proven unsatisfactory for certain applications where heat services are not readily available for expanding the shrinkable sleeve or where sleeves having sufficient shrinkage characteristics are not readily available.
Other devices for coupling hoses to nipples, include a single conically shaped nipple received in the free end of a hose and having a sleeve drawn thereover to compress the hose between the sleeve and nipple. A device of this type is shown in U.S. Pat. No. 2,821,775. While satisfactory for its intended purpose, such a coupling would not be satis factory for subterranean plastic natural gas pipes.